Method for making a PEG phase change composite

ABSTRACT

A heat energy storage system may have a shape-stabilized composite prepared using an easy impregnation method involving a porous Ca 2+ -doped MgCO 3  matrix and PEG as the functional phase. The heat storage capability, microstructures, and interactions with the PEG/CaMgCO 3  composite can be characterized by DSC, SEM imaging, FT-IR spectroscopy, and TGA. Likely because of the synergistic phase change effect of CaMgCO 3  and PEG, the PEG/CaMgCO 3  composites can have high thermal enthalpies, and their enthalpy efficiencies are substantially higher than those of traditional shape stabilized PCMs. The functional material PEG can permeate porous CaMgCO 3  matrices under capillary action. Liquid PEG can be stabilized within the porous matrix, and/or the CaMgCO 3  matrix can improve the thermal stability of the PEG. The high heat energy storage properties and good thermal stability of such organic-inorganic composites offers utility in a range of applications, including thermal energy storage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of Ser. No. 16/047,593, nowU.S. Pat. No. 10,626,238, having a filing date of Jul. 27, 2018.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to phase change materials (PCM) employinginorganic-organic composites, particularly PCMs useful in heat storagedevices.

Description of the Related Art

A variety of materials, organic, inorganic, or mixtures thereof, canstore ambient heat and release it when necessary by changing phaseduring temperature shifts. Such storage presents an opportunity forsolar energy management and utilization. Phase change materials (PCMs)are important, and often commercially available, heat storage materialswith a high density of latent heat and the capacity to maintain aconstant temperature during heat release. PCMs can be used, for example,in smart housing, solar energy installations, temperature-controlledgreenhouses, temperature-regulating textiles, electronics heatmanagement, and the like. PCMs typically undergo a phase change nearroom temperature and/or have a high latent for utility in theaforementioned applications.

Polyethylene glycol (PEG) is useful for heat storage applications. PEGis low cost and offers good chemical stability, no phase segregation, aninsignificant degree of supercooling, no toxicity, and good thermal andchemical stability properties. PEG is reusable, biocompatible, andundergoes a small volume change during phase transitions compared withsolid-solid PCMs. Moreover, PEG has a melting temperature range that canbe tuned in a range of from 32° C. to 60.7° C., depending on the PEGmolecular weight. PEG is resistant to erosion, has a low vapor pressure,can be directly incorporated into carrier matrices, and is thermallystable.

However, PEG exhibits two unavoidable drawbacks. First, liquid PEG canleak during phase transition. Leaked liquid PEG can contaminatesurrounding materials and jeopardize the function of a system. Second,PEG has a low thermal conductivity, which precludes its use in certainapplications.

Inorganic PCMs have higher phase change enthalpies compared to organiccompounds. Lithium and sodium carbonate composite PCM have been reportedto have high thermal conductivities and an energy storage density over530 kJ/kg. Lithium nitrate showed an energy storage density 357 J/g andsodium nitrate, 222 J/g. These hydrate salts are known to havehigh-energy storage density, but their supercooling properties are asignificant drawback for practical application.

CN 107954671 A by Hu discloses a raw material for preparing a phasechange thermostated diatomaceous plate, a phase changetemperature-regulating diatomaceous plate, and a preparation methodthereof. Hu's raw material comprises a separately packaged first slurry,a second slurry, and a binder. Hu's first slurry comprises adiatomaceous earth (amorphous silica, opal, SiO₂.nH₂O, also referred toas diatomite or kieselguhr) phase change composite material, a calciummaterial, a filler, and water. Hu's second slurry contains diatomaceousearth, a calcium material, active magnesium oxide, a filler, and water.In the first slurry and the second slurry, the liquid to solid ratio isindependently (3 to 4): 1. Hu's organic phase change material isimmersed in the pores of the modified diatomaceous earth to load theorganic phase change material (PCM), ensuring the stable combination ofthe phase change material and the diatomaceous earth, and avoid leakageof the PCM. The first and second slurries simultaneously containdiatomaceous earth, and the two have good adaptability, and the layerstructure is bonded by an adhesive to form a phase changetemperature-controlled diatom plate, without using a special packagingcontainer. Packaging of Hu's phase change temperature regulatingmaterial can avoid leakage of the PCM.

However, Hu appears to require a silicon oxide compound in at least 70wt %, as well as a modified diatomite with an amino group bonded to itssurface. Hu also requires a binder, which Hu teaches may be polyvinylalcohol, polyethylene glycol, and polyethylene oxide. Hu does notindicate that the binder penetrates pores of its plates. Instead, Hu'sorganic phase change material is a composite of n-butyl stearate andmethyl stearate or a paraffin wax, which may penetrate pores of thediatomaceous earth. Moreover, Hu does not describe a phase changecomposite of PEG encapsulated in a porous matrix of calcium ion dopedmagnesium carbonate.

CN 106701034 A by Wu discloses a solid composite phase-change thermalstorage material comprising water, sodium carbonate, potassiumcarbonate, magnesium oxide, kieselguhr, quartz sand, and kaolin. Kaolinis a clay mineral with the chemical composition Al₂Si₂O₅(OH)₄, which isa layered silicate mineral, with one tetrahedral sheet of silica (SiO₄)linked through oxygen atoms to one octahedral sheet of alumina (AlO₆)octahedra. Wu discloses preparing the solid composite phase-changethermal storage material involving: stirring, rubbing sand, filtering,performing hydraulic molding, and performing sintering molding. Wufurther discloses that the thermal storage density is improved to twicethat of a common material, the thermal storage material can endure hightemperature, can be heated to be greater than 650° C., and is not liableto efflorescence when being used for a long time.

However, Wu's inorganic component requires aluminum and/or siliconoxides, rather than, e.g., alkaline earth metal ions. Wu also requiresat least 25 wt %, and preferably 34 to 45 wt %, MgO as a portion of itsinorganic composition. Although Wu describes using conventional organicheat storage materials include higher aliphatic hydrocarbons, aromatichydrocarbons, alcohols, carboxylic (fatty) acids, paraffin, etc.,optionally including polyethylene glycol (PEG), Wu does not describe aphase change composite of PEG encapsulated in a porous matrix of analkaline earth metal carbonate, particularly magnesium carbonate, andmore particularly a calcium ion doped magnesium carbonate.

WO 2017/206563 A1 by Chen discloses a phase change energy storage powderand its preparation, relating to the field of building energy-savingmaterials. Chen discloses a powder type energy storage materialcomprising an anti-supercooling agent prepared by extrusion claddingusing a screw. Chen's phase change energy storage powder is preparedfrom the following components in parts by weight: 80 to 85 parts of aphase change material, 2 to 3 parts of an active agent, 0.1 to 0.3 partsof an anti-supercooling agent, 8 to 12 parts of a coating agent, 1 to 2parts of a heat conduction agent, 0.5 to 1 parts of a stabilizer, and 2to 3 parts of a flame-retardant agent. Chen's phase change energystorage powder is reported to have high energy density, not besupercooled, and be flame-retardant and highly compatible with most ofbuilding materials. According to Chen, the powder can be added in largequantity without affecting the strength of a building material, and canbe applied to concrete, mortar, putty, paint, or the like.

However, Chen's phase change material is calcium chloride hexahydrate,sodium sulfate decahydrate, disodium hydrogen phosphate dodecahydrate,and/or sodium acetate trihydrate. Although Chen may describe using notmore than 5%, of an active agent of glycerol fatty acid ester(s) and/orpolyoxyethylene fatty acid glyceride(s), an encapsulated agent ofethylene-vinyl acetate copolymer, polycaprolactone, polyethylene wax,polypropylene wax, and/or rosin, Chen does not describe a phase changecomposite of inorganic encapsulated PEG PCMs, nor carbonate PCMs, muchless composites thereof, particularly a PEG encapsulated porous matrixof calcium ion doped magnesium carbonate.

CN 107365121 A by Yang discloses an inorganic material coatedphase-change microcapsule compounded phase-change putty as well as apreparation method and application thereof. Yang's inorganic coatedphase-change microcapsule compounded phase-change putty comprisescalcium magnesium powder, Portland cement, ash calcium powder andadditives. Yang's inorganic material coated phase-change microcapsulesare made of a nonflammable material, so that the inorganic materialcoated phase-change microcapsules are relatively high in safety whenbeing compared with an organic material coated phase-changemicrocapsules. The inorganic material coated phase-change microcapsulecompounded phase-change putty maintains properties of the originalputty, furthermore has an indoor temperature conditioning function, andhas the characteristics that indoor temperature fluctuation is reducedand energy consumption is reduced. The indoor temperature of a housewith inner walls made of the phase-change putty can be controlled to 2-4DEG C, and thus a good heat-preservation temperature is achieved

However, Yang's inorganic component comprises 25 to 35 wt. % CaCO₃ and15 to 20% MgCO₃, and it may include an anti-foaming agent selected fromthe group consisting of polyether, silicone, and alcohol antifoamingagents, such as GP type glycerol polyether, GPE type polyoxyethylenepolyoxypropylene glyceryl ether, etc.. However, Yang additionallyrequires at least one of SiO₂, TiO₂, SnO₂, ZrO₂, and Al₂O₃. Also, Yangselects an organic portion from low MW alcohols, organic acids, esters,and alkanes, such as glycerol, butanol, dodecanol, tetradecanol, cetylalcohol, and/or erythritol; caprylic acid, capric acid, dodecanoic acid,myristic acid, palmitic acid, octadecanoic acid, lauric acid, myristicacid, palmitic acid, and/or stearic acid; cellulose laurate and/or cetylstearate; or a paraffin wax (MP: 14 to 80° C.), aromatic hydrocarbon,and/or aliphatic hydrocarbon (carbon number: 8 to 100). Yang requires inits PCMs a portion of Portland cement, a hydraulic material with atleast ⅔ by mass of calcium silicates and a remainder of Al- andFe-containing clinker phases and other compounds, i.e., (CaO)₃.SiO₂ (45to 75%), (CaO)₂.SiO₂ (7 to 32%), (CaO)₃.Al₂O₃ (0 to 13%),(CaO)₄.Al₂O₃.Fe₂O₃ (0 to 18%), and CaSO₄.2H₂O (2 to 10%), whereintypical components by percent are CaO (61 to 67%), SiO₂ (19 to 23%),Al₂O₃ (2.5 to 6%), Fe₂O₃ (0 to 6%), and SO₃ (1.5 to 4.5%). Yang's ratioof CaO to SiO₂ is at least 2.0, and its MgO content is below 5.0 wt %.Yang does not describe PEG or similar polymers, and, while describingMgCO₃ and CaCO₃ separately, Yang fails to explicitly describe Ca²⁺-dopedMgCO₃. Further, Yang is silent on such a porous matrix of calcium iondoped magnesium carbonate, encapsulating PEG in a PCM.

WO 2013/077379 A1 by Morita discloses a heat storage material thatmaintains a stable particle diameter even with repeated phase changes,capable of withstanding long-term use. Morita's heat storage material isobtained by dispersion of particles containing a heat storage substanceand an elastomer. Morita's heat storage material comprises at least oneheat storage substance selected from paraffin compounds, fatty acids,ester compounds of fatty acids, aliphatic ethers, aliphatic ketones, andaliphatic alcohols.

However, Morita requires an elastomer with a molecular weight of atleast 10,000 g/mol, as well as a paraffin, fatty acid, aliphatic ether,aliphatic ketone, or fatty alcohol as heat storage material. WhileMorita may disclose the use of a pegylated-ether surfactant,polyoxyethylene stearyl ether, Morita does not indicate its chain lengthnor does Morita employ its surfactant in heat storage—rather to emulsifyits heat storage material. Although Morita may allow the presence ofCaCO₃ or MgCO₃ as optional fillers, Morita fails to describe using MgCO₃as a PCM, particularly not as a composite of PEG encapsulated in aporous matrix of calcium ion doped MgCO₃.

Karaman et al.'s article in Solar Energy Materials and Solar Cells 2011,95(7), 1647-1653, discloses the preparation, characterization, anddetermination of thermal energy storage properties of polyethyleneglycol (PEG)/diatomite (also referred to as diatomaceous earth orkieselguhr) composite as a novel form-stable composite phase changematerial (PCM). Karaman's composite PCM was prepared by incorporatingPEG in the pores of diatomite. The PEG could be retained by 50 wt % intopores of the diatomite without the leakage of melted PEG from thecomposite. Karaman characterizes its composite PCM using SEM and FT-IRanalysis, and determines thermal properties of the composite PCM bydifferential scanning calorimetry (DSC). DSC results showed that themelting temperature and latent heat of Karaman's composite PCM are27.70° C. and 87.09 J/g, respectively. Thermal cycling test on thethermal reliability of Karaman's composite PCM showed that Karaman'scomposite PCM had good thermal reliability and chemical stability.Thermogravimetry (TG) analysis showed that the impregnated PEG into thediatomite had good thermal stability. Thermal conductivity of Karaman'scomposite PCM was improved by adding expanded graphite in different massfractions. Thermal energy storage of Karaman's composite PCM was alsotested.

However, Karaman does not teach the use of MgCO₃, doped or otherwise, ina PCM and more particularly fails to describe a phase change compositeof PEG encapsulated in a porous matrix of calcium ion doped magnesiumcarbonate.

Gutierrez et al. in Applied Energy 2015, 154, 616-621, disclosesevaluating bischofite (hydrous magnesium chloride mineral with formulaMgCl₂.6H₂O), a by-product of the non-metallic mining industry, as phasechange material in thermal energy storage. Gutierrez indicates thatbischofite shows little cycling stability, therefore studies the mixturewith an additive. Gutierrez discloses that polyethylene glycol (PEG) isa PCM itself, and uses PEG (with different molecular weights) as anadditive in a bischofite PCM to improve its thermal behavior.

However, Gutierrez does not teach the use of MgCO₃, doped or otherwise,in a PCM and more particularly fails to describe a phase changecomposite of PEG encapsulated in a porous matrix of calcium ion dopedmagnesium carbonate.

Hao et al.'s article in Thermochimica Acta 2015, 604, 45-51 disclosessynthesizing porous MgO material with ultrahigh surface area. Acomposite PCM was prepared from PEG-1000 and the porous MgO, the phasechange temperatures and enthalpy of the composite were measured andanalyzed,. Hao's mesoporous magnesium oxide (MgO) material wassynthesized using an integration of the evaporation-induced surfactantassembly and magnesium nitrate pyrolysis. Hao's as-prepared MgO materialis crystalline, and possesses three-dimensional interconnected mesoporesand a surface area as high as 596 m²/g. Hao fabricates ashape-stabilized phase change composite of PEG/MgO using the porous MgOas a matrix and polyethylene glycol (PEG-1000) as the functional phasefor heat energy storage. Compositions and microstructures of Hao'sPEG/MgO composite were determined by Fourier transformation infraredspectroscope (FT-IR), X-ray diffractometer (XRD), scanning electronicmicroscope (SEM), and thermogravimetric analysis (TGA). Phase changeproperties of Hao's PEG/MgO composite were determined by differentialscanning calorimeter (DSC).

However, Hao does not teach the use of MgCO₃, doped or otherwise, in aPCM and fails to describe a phase change composite of PEG encapsulatedin a porous matrix of calcium ion doped magnesium carbonate.

Wen et al in J of Thermal Analysis and calorimetry 2018, 132(3),1753-1761 discloses using bone char (BC, consisting mainly of Ca₃(PO₄)₂or hydroxyapatite (Ca₅(PO₄)₃(OH)) in 57 to 80%, CaCO₃ in 6 to 10%, andcarbon in 7 to 10%) as a porous material useful for preparing aform-stable composite phase change material (PCM). A polyethylene glycol(PEG 6000)/BC composite PCMs was prepared by impregnation. The PEG wasused as the phase change material, and two different particle sizes ofBC (0.8-1 mm: BC-1; 0.25-0.8 mm: BC-2) were used as the supportingmaterials. The phase composition and chemical structure of Wen'scomposite PCMs (PEG/BC-1 and PEG/BC-2) were characterized using X-raydiffraction and Fourier transformation infrared. Thermal properties andthermal stability of Wen's composite PCMs were determined bydifferential scanning calorimeter (DSC) and thermogravimetric analysis(TGA). DSC results showed that the maximum impregnation percentage forPEG into BC-1 and BC-2 was 38.77 and 43.91%, respectively, withoutmelted PCM seepage from the composites. The TGA analysis revealed thatWen's composite PCMs had good thermal stability above their workingtemperature range. The thermal cycle test of 100 melting-freezing cyclesshowed that the composite PCMs have good thermal reliability andchemical stability. The form-stable composite PCMs can be used asthermal energy storage material for waste heat storage and solar heatingsystem.

However, Wen does not teach the use of MgCO₃, doped or otherwise, in aPCM and more particularly describe a phase change composite of PEGencapsulated in a porous matrix of calcium ion doped magnesiumcarbonate.

MgCO₃ is difficult to prepare properly under ambient conditions. Knownmethods of synthesizing MgCO₃, e.g., produce MgCO₃ successfully, havecertain deficiencies. For example, microscale particles from thesemethods may be too small to satisfy the basic requirements ofmicro-analysis, including micro-constitution and micro-compositionanalysis. Thus, new methods of MgCO₃ synthesis with simplified processesand good efficiency are worth exploring.

PEG leakage problem is a significant problem that has remained unsolvedby encapsulating PEG in a metal or alloy for reason includingencapsulation alone can cause problematic supercooling. Moreover, PCMencapsulation is a complicated and laborious process. By contrast, shapestabilization is advantageous for the preparation of PCMs, causinglittle or no supercooling, and avoiding cell formation. Shapestabilization processing materials with a high compatibility and goodabsorption may be used to capture a PCM and guarantee the stability andeven distribution of a PCM. Shape stabilization can be low cost andapplicable to higher percentage of PCM than encapsulation. However,known inorganic porous support-based shape stabilization processes havebeen limited, mainly relying on noble inorganic materials.

Aspects of the invention may address one or more of the above-describedshortcomings of the art.

SUMMARY OF THE INVENTION

Aspects of the invention provide a phase change composite, comprising:an organic component comprising, based on total organic mass, at least75 wt % of a thermoplastic polymer; and an inorganic porous matrixcomprising, based on total inorganic mass, at least 50 wt % calcium iondoped magnesium carbonate, wherein at least a portion of the organiccomponent is encapsulated in the porous matrix. All modifications andsubstitutions as detailed herein, except where otherwise specified, maybe combined or permuted within the scope of the invention

The thermoplastic polymer may comprise a polyether, such as polyethyleneglycol, polypropylene glycol, polyoxetane, or a mixture of two or moreof any of these, preferably at least polyethylene glycol, especiallyPEG-6000. The thermoplastic polymer may have a number-average molecularweight in the range of from 1,000 to 50,000, 1,000 to 50,000, 2,500 to10,000, or 4,000 to 8,000. The organic component may comprise at least90 or 95 wt. % or more polyethylene glycol.

The porous matrix may comprise from 2.5 to 20, 5 to 17.5, 10 to 15, orat least 10 mol % calcium ions. The inorganic mass may comprise at least90 wt % calcium ion doped magnesium carbonate. The inorganic porousmatrix may have a BET specific surface area in a range of from 14 to 20m²/g.

Composites according to the invention may have an apparent meltingenthalpy in a range of from 400 to 600 J/g. Composites within the scopeof the invention may have a thermal storage efficiency in a range offrom 125 to 250%.

Composites within the scope of the invention may have no visibleimpurities in their DSC scans after at least 50 cycles.

Aspects of the invention provide heat storage systems having one or moreof any of the phase change composites described herein.

Aspects of the invention provide methods of preparing any inventivephase change composite described herein, preferably a compositecomprising polyethylene glycol (PEG) encapsulated in a porous matrix.The method may comprise: dissolving magnesium nitrate, calcium nitrates,and ammonium carbonate in water to form a solution; stirring thesolution at constant pH in the range of 7.5 to 9.5 for 6 to 18 hours;heating the solution at a temperature in the range of 150 to 250° C. for12 to 48 hours to produce mesoporous calcium ion doped magnesiumcarbonate; mixing the mesoporous calcium ion doped magnesium carbonatewith an alcoholic solution of PEG and stirring for a time in the rangeof 2 to 8 hours; and evaporating the alcohol to produce the composite ofPEG encapsulated in a mesoporous matrix calcium ion doped magnesiumcarbonate.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A (a) shows an x-ray diffraction (XRD) pattern of MgCO₃, (b) showsan XRD pattern of 5 mol % Ca²⁺-doped MgCO₃; (c) shows an XRD pattern of10 mol % Ca²⁺-doped MgCO₃; (d) shows an XRD pattern of 15 mol %Ca²⁺-doped MgCO₃;

FIG. 1B (e) shows an XRD pattern of pure PEG-6000; (f) shows an XRDpattern of 10 mol % Ca²⁺-doped MgCO₃ impregnated with PEG-6000; (g)shows an XRD pattern of 15 mol % Ca²⁺-doped MgCO₃ impregnated withPEG-6000;

FIG. 2A (a)-(d) respectively show Fourier transform-infra red (FT-IR)spectra of (a) MgCO₃, (b) 5 mol % Ca²⁺-doped MgCO₃, (c) 10 mol %Ca²⁺-doped MgCO₃, (d) 15 mol % Ca²⁺-doped MgCO₃,

FIG. 2B (e) PEG-6000, (f) PEG-6000/MgCO₃, (g) PEG-6000/5 mol %Ca²⁺-doped MgCO₃, (h) PEG-6000/10 mol % Ca²⁺-doped MgCO₃, and (i)PEG-6000/15 mol % Ca²⁺-doped MgCO₃;

FIG. 3A shows a field emission scanning electron microscopy (FE-SEM)image of MgCO₃;

FIG. 3B shows a field emission scanning electron microscopy (FE-SEM)image of 5 mol % Ca²⁺-doped MgCO₃;

FIG. 3C shows a field emission scanning electron microscopy (FE-SEM)image of 10 mol % Ca²⁺-doped MgCO₃;

FIG. 3D shows a field emission scanning electron microscopy (FE-SEM)image of 15 mol % Ca²⁺-doped MgCO₃;

FIG. 4 shows N₂ adsorption-desorption isotherms of 10 mol % Ca²⁺-dopedMgCO₃ and 15 mol % Ca²⁺-doped MgCO₃ samples;

FIG. 5 shows pore size distributions of 10 mol % Ca²⁺-doped MgCO₃ and 15mol % Ca²⁺-doped MgCO₃ samples;

FIG. 6A shows x-ray photoelectron spectroscopy (XPS) spectra of surveyspectrum 15 mol % Ca²⁺-doped MgCO₃;

FIG. 6B shows x-ray photoelectron spectroscopy (XPS) spectra ofmagnesium is region;

FIG. 6C shows x-ray photoelectron spectroscopy (XPS) spectra of calcium3p region;

FIG. 6D shows x-ray photoelectron spectroscopy (XPS) spectra of oxygenis region;

FIG. 6E shows x-ray photoelectron spectroscopy (XPS) spectra of carbonis region;

FIG. 7 shows thermogravimetric analysis (TGA) curves of PEG-6000, 15 mol% Ca²⁺-doped MgCO₃, and PEG-6000/15 mol % Ca²⁺-doped MgCO₃ samples;

FIG. 8A shows melting-freezing DSC curves of PEG-6000;

FIG. 8B shows melting-freezing DSC curves of PEG-6000/10 mol %Ca²⁺-doped MgCO₃PCMs;

FIG. 9A shows an FE-SEM image of a 15 mol % Ca²⁺-doped MgCO₃PCM sample;

FIG. 9B shows melting-freezing DSC curves the 15 mol % Ca²⁺-dopedMgCO₃PCM sample from FIG. 9A;

FIG. 10A shows DSC heating curve of 15 mol % Ca²⁺-doped MgCO₃

FIG. 10B shows DSC heating curve of PEG-6000;

FIG. 10C shows DSC heating curve of a PEG-6000/15 mol % Ca²⁺-dopedMgCO₃;

FIG. 11A shows photographs of pure PEG-6000 heated at 80° C. fordifferent lengths of time;

FIG. 11B shows photographs of a PEG-6000/15 mol % Ca²⁺-doped MgCO₃composite PCM heated at 80° C. for different lengths of time;

FIG. 12A shows an FE-SEM image of a PEG-6000/15 mol % Ca²⁺-doped MgCO₃composite PCM;

FIG. 12B shows melting-freezing DSC curves of the PEG-6000/15 mol %Ca²⁺-doped MgCO₃ composite PCM;

FIG. 13A shows compatibility test photographs of (a) tin, (b) aluminum,and (c) copper metal sheets embedded in PEG-6000/15 mol % Ca²⁺-dopedMgCO₃ samples; and

FIG. 13B respectively show compatibility test photographs of the samplesfrom FIG. 13A (a)-(c) after two months under atmospheric conditions inDammam, Saudi Arabia (high temperature July-August˜50° C., lowesttemperature ˜10° C.).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aspects of the invention phase change composites which may comprise: anorganic component comprising, based on total organic mass, at least 75,80, 85, 90, 92.5, 95, 96, 97, 98, 99, 99.5, or 99.9 wt % of athermoplastic polymer; and an inorganic porous matrix comprising, basedon total inorganic mass, at least 50, 60, 75, 85, 90, 92.5, 95, 96, 97,98, 99, 99.5, or 99.9 wt % calcium ion doped magnesium carbonate,wherein at least a portion of the organic component is encapsulated inthe porous matrix. “Organic mass,” as used herein, does not includecarbon that is contained in ionic counter ions of inorganic materialssuch as carbonate.

Thermoplastic polymers within the scope of the invention may comprise apolyether, such as polyethylene glycol, polypropylene glycol,polyoxetane, or a mixture of two or more of any of these, preferably atleast polyethylene glycol, especially PEG-6000. Useful polymers mayinclude polyethers, polyesters, polyolefins, polyurethanes,poly(meth)acrylates, polyamides, polyimides, and even sparinglycross-linked elastomers. It may also be desirable to at least partiallyexclude elastomers in particular, and any other named polymer class ingeneral. Particularly useful polymers are presently include polyethers,including polyethylene glycol (PEG), polypropylene glycol (PPG),polyoxetane, poly-1,2-butylene glycol, poly-tetrahydofuran, etc.. Formsof PEG useful in the inventive composites may be any chain length,though preferably at least 1000 g/mol, such as PEG-1000, PEG-1450,PEG-1500, PEG-2000, PEG-3000, PEG-3350, PEG-4000, PEG-5000, PEG-6000,PEG-8000, PEG-10000, PEG-12000, PEG-20000, or PEG-25000, for example.

Useful thermoplastic polymers may have a number-average molecular weightin the range of from 1,000 to 50,000, 1,000 to 50,000, 2,500 to 10,000,or 4,000 to 8,000, which may be tailored by application. Inventivecomposites may, but need not, exclude monomeric organic acids and mayuse higher MW materials than paraffin waxes, e.g., of at least 1,000,1,250, 1,500, 1,750, 2,000, 2,500, 3,000, 4,000, or 5,000 g/molsubstantially thermoplastic polymers. Useful molecular weights,depending upon application may be in a range of from 1,000 to 1,000,000,1,250 to 750,000, 1,500 to 500,000, 2,000 to 250,000, 2,500 to 200,000,3,000 to 175,000, 3,500 to 150,000, 4,000 to 125,000, 4,500 to 100,000,5,000 to 75,000, 5,250 to 50,000, or 5,500 to 35,000 g/mol. A preferableMW may be less than 10,000 g/mol.

The organic component may comprise at least 50, 65, 75, 85, 90, 92.5,95, 96, 97, 98, 99, 99.5, or 99.9 wt. % or more of the thermoplasticpolymer(s), particularly polyethylene glycol. Composites within thescope of the invention may comprise at least 5, 7.5, 10, 12.5, 15, 17.5,or 20 wt % polymer component, based on the total composite mass, andwill generally comprise less that 75, 65, 50, or 40 wt % polymer. Usefulpolymers will generally have a polydispersity index (PDI) in a range offrom 1.1 to 7, 1.2 to 5, 1.25 to 4, or 1.3 to 3, and generally under 10,though it may exceed 10 under the appropriate binding/adhesionconditions.

The porous matrix may comprise from 0.5 to 40, 1 to 30, 2.5 to 20, 5 to17.5, 10 to 15 mol % alkaline earth metal ions, esp. calcium ions, or atleast 5, 7.5, 10, 12.5, 15, 17.5, or 20 mol % alkaline earth metal ions,and may have no more than 75, 50, or 25 mol % alkaline earth metal ions.Inorganic components of composites according to the invention generallyinclude alkaline earth metal ions, particularly Mg²⁺, Ca²⁺, Sr²⁺, and/orBa²⁺, which are generally present as salts, particularly carbonatesalts, and/or as dopants in carbonate salt lattices. The inorganiccomponent may be at least 50, 60, 70, 80, 85, 90, 91, 92, 93, 94, 96,97, 98 or more wt % calcium ion doped magnesium carbonate

Based on the total inorganic mass, composites within the scope of theinvention may have less than 70, 50, 40, 33, 25, 20, 15, 12.5, 10, 7.5,5, 4, 3, 2.5, 2, 1, or even 0.5 wt % of SiO₂, TiO₂, SnO₂, ZrO₂, and/orAl₂O₃ amongst the inorganic components, and may even be devoid of anysuch oxides (including silicates) beyond inevitable traces. Compositeswithin the scope of the invention may largely exclude, e.g., less than10, 5, 2.5, or 1 wt %, or completely exclude, e.g., kaolin, Portlandcement, bischofite, hydroxyapatite, and/or diatomaceous earth in theirinorganic matrices, though these materials can be tolerated as fillersand/or diluents. No, or at least less than 10, 5, 2.5 wt %,surface-modified inorganic materials, i.e., covalently bonded surfacemodifications, such as attaching functional groups, are needed incomposites within the scope of the invention, though surface-modifiedmaterials may be used, as desired, or limited to no more than 50, 40,33, 25, 10, 5, or 2.5 wt % of the inorganics. The inorganic portion inthe composites within the scope of the invention may limit MgO to lessthan 15, 10, 5, 2.5, 1, or 0.5 wt % of the inorganics. The organicportion in the composites within the scope of the invention may limitelastomeric and/or cross-linked components to less than 15, 10, 5, 2.5,1, or 0.5 wt % of the organic portion. Although the composite can takemany physical forms, it will preferably be a non-fluid and/ornon-emulsified solid.

Synthetic Method (Hydrothermal Method)

Composites within the scope of the invention can be made using, e.g.,reagent-grade polyethylene glycol with an average molecular weight of6000 (PEG-6000, available from the Guangzhou Chemical Agent Company(Guangzhou, China)); Ca²⁺ nitrate salts, magnesium nitrate hexahydrate(Mg(NO₃)₂.6H₂O), and abs. ethanol (available from Shanghai SinopharmChemical Reagent Co., Ltd.); and ammonium carbonate (available fromSigma-Aldrich). Other alkaline earth metal ions may alternately be usedsuch as Be²⁺, Ca²⁺, Sr²⁺, and/or Ba²⁺. Further cations, such as alkalimetals (e.g., Na⁺, K⁺, Li⁺, and/or Cs⁺), and/or cations of Ti, Zr, Cr,Mo, Mn, Fe, Ru, Os, Co, Rh, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, and/or Sn(particularly Mn, Fe, CO, Ni, Cu, Ag, Zn, and/or Cd) may be addeddepending upon the desired application. These cations are typicallyintroduced in forms which are water soluble but wherein the metal cationcan be separated, preferably by (co)precipitation from the anion.Examples of useful anions may be halides, such as chlorides or bromides,nitrates, citrates, tartrates, or the like.

Appropriate amounts of Mg²⁺ and Ca²⁺ nitrate salts can be dissolvedseparately in deionized water. Exemplary molar ratios of Ca²⁺ to Mg²⁺may be in ranges having lower endpoints of 1:100, 1:50, 1:30, 1:25,1:20, or 1:10, and/or upper end points of 1:1, 1:2, 1:3, 1:4; 1:5, 1:6,1:7, 1:8, 1:9, or 1:10. A corresponding amount of a (NH₄)₂CO₃ solution(4.1 mol/cc, available from Merck) can be added to the Mg²⁺ and Ca²⁺+nitrate solutions to co-precipitate the metallic ions. The pH of thesolutions can be maintained at 8.5 and/or adjusted to a specific valueby adding NH₃.H₂O (25 wt %) or HNO₃ (1 M). Useful pHs are usually atleast neutral or basic, but may be in a range of from 6.5 to 10.5, 7 to10, 7.1 to 9.5, 7.5 to 9, 8 to 8.8, 8.25 to 8.75, or 8.4 to 8.6. Thesolutions are preferably kept at pH ˜8.5 and stirred vigorously 12hours. This stirring may be conducted for more or less time asdetermined by the reaction extent, but acceptable reaction times may bein a range of from 5 to 24, 8 to 16, or 10 to 14 hours. Each precursorsuspension is transferred to a plastic container with an inner volume of500 cm³. The plastic container is inserted into a steel vessel, and themouth of the vessel is closed. The hydrothermal reaction then proceedsat 200° C. over 24 hours. The hydrothermal reaction should generally beconducted at least at 160° C. for at least 24 hours, but acceptablereaction temperature ranges are from 150 to 280, 160 to 260, 175 to 250,185 to 230, or 190 to 225° C. Reaction times at any of thesetemperatures may be at least 18, 20, 24, 28, 30, 36, or 48 hours and maybe terminated, after completion of the reaction, which maybe before 7,6, 5, 4, 3, 2.5, 2, 1.5, or 1 day. The resulting powders are washed withdeionized water and alcohol, followed by drying at 120° C. The shape,size, and make-up of the inorganic component can be adjusted by varyingsynthesis conditions, including pH, temperature, pressure, and chemicalcompositions. Potential inorganic crystal shapes and sizes are discussedbelow.

An exemplary preparation of a shape-stabilized composite PCM is asfollows. 0.5 g of PEG-6000 was dissolved in 80 mL absolute ethanol. 0.2g of mesoporous 15 mol % Ca²⁺-doped MgCO₃ was then added to theethanolic PEG-6000 solution. After stirring for 4 h, the ethanol wasevaporated at 80° C. for 24 h. The PEG-6000/15 mol % Ca²⁺-doped MgCO₃composite PCM was collected for further characterization.

The hydrothermal method of manufacturing the inorganic component of thecomposites within the scope of the invention can involve, under alkalineconditions, hydrolyzing urea (or (NH₄)₂CO₃) to yield NH₄ ⁺ and CO₃ ²⁻,as indicated in Equation 1 below.CO(NH₂)₂+2H₂O→2⁻NH₄+CO₃ ²⁻  Eq. 1

Under the hydrothermal conditions, Mg²⁺, Ca²⁺, OH⁻, CO₃ ²⁻, NO₃ ²⁻, andNH₄ ⁺ coexist in the aqueous reaction system. The temperature is slowlyraised from 25 to 200° C., and the pH is preferably adjusted to 8.5. NO₃⁻ ions can be removed, e.g., by filtration after competitivecrystallization. CaMg₅(CO₃)₄(OH)₂.4H₂O can be obtained at 80° C. in thepresence of Mg²⁺, Ca²⁺, OH⁻, and CO₃ ²⁻. Moreover, Mg₅(CO₃)₄(OH)₂.4H₂O,Mg(OH)₂, and Ca(OH)₂ can be observed at 160° C. for reaction timesshorter than 30 hours. CaMg₅(CO₃)₄(OH)₂.4H₂O may provide magnesium forsynthesizing MgCO₃, and Ca²⁺ may be substituted into MgCO₃. However, atreaction temperatures above 160° C. and reaction times greater than 30hours, CaMg₅(CO₃)₄(OH)₂.4H₂O generated during this process may beconverted into MgCO₃. Relevant chemical reactions may be represented byEquations 2 to 4, below.CO(NH₂)₂+Mg²⁺+2H₂O→MgCO₃↓+2NH₃+H₂↑  (Eq. 2)CO₂+H₂O→CO₃ ²⁻+2H⁺  (Eq. 3)Mg²⁻+CO₃ ²⁻→MgCO₃  (Eq. 4)

At 160° C., the Gibbs free energy of the reaction)(Δ_(f)G°) is −469.41kJ/mol, i.e., the reaction is thermodynamically spontaneous at thereaction temperature. This reaction was exothermic, with an enthalpy ofΔ_(f)H°=−309.17 kJ/mol. Consequently, this reaction is thermodynamicallypredicted to proceed readily at 160° C.

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

FIG. 1A(a) shows the XRD pattern of MgCO₃ synthesized as describedherein. FIG. 1A (b) shows the XRD pattern of 5 mol % Ca²⁺-doped MgCO₃.FIG. 1A(c) shows the XRD pattern of 10 mol % Ca²⁺-doped MgCO₃. FIG.1A(d) shows the XRD pattern of 15 mol % Ca²⁺-doped MgCO₃. The samples inthese figures were identified by XRD as single-phase MgCO₃ without anyimpurities. The lattice constants calculated from the pattern (a=4.63,Å, c=14.93 Å) were compatible with reported values (a=4.633 Å, c=15.015Å). The diffraction peaks could be indexed as hexagonal phaserhomb-centered MgCO₃, in good agreement with the Joint Committee onPowder Diffraction Standards (JCPDS) No. 08-0479.

The intensities of the sharp peaks in FIG. 1A(a) to (d) indicate thatthe MgCO₃ was highly crystalline. The addition of Ca²⁺ had nosignificant effect on the MgCO₃ crystal structure, as shown in FIG.1A(a) to (d). The XRD patterns remain substantially identical, evenafter the addition of 15 mol % Ca²⁺, compared to patterns collected fromMgCO₃ alone. The XRD patterns displayed the same peaks in FIG. 1A(c),after the addition of 10 mol % Ca²⁺, and in FIG. 1A(d), after theaddition of 15 mol % Ca²⁺. CaCO₃ and MgCO₃ reportedly have the samestructure as MnCO₃, FeCO₃, and (Mn,Fe)CO₃. However, the substitution ofCa²⁺ for Mg²⁺ in MgCO₃, or of Mg²⁺ for Ca²⁻ in CaCO₃, produces anordered CaMg(CO₃)₂ structure that accommodates the difference betweenthe ionic radii of Ca²⁺ (r=1.14 A) and Mg²⁺ (r=0.86 A). The newstructure forms alternating layers with Ca²⁺ and Mg²⁺ ions. In the Ca²⁺layer, the Ca—O interatomic distance is 2.390 A, and in the Mg layer,the Mg—O distance is 2.095.

The X-ray diffraction (XRD) patterns of the samples were obtained outusing an X-ray diffractometer (Shimadzu XRD-6000) with graphitemonochromatized Cu-Kα radiation, operated at 40 kV and 30 mA. Themorphology and microstructures of the MgCO₃ material and PEG/MgCO₃composite were observed using a field emission scanning electronmicroscope (FESEM, JEOL JSM-6700F, Japan). The specific surface area andpore size distribution of the mesoporous 15 mol % Ca²⁺-doped MgCO₃material were measured using a specific surface area analyzer(Micromeritics ASAP 2020 V4.01 E, USA). The Fourier transform infrared(FT-IR) spectra were recorded using a Thermo Nicolet 380 from 400 to4000 cm⁻ using KBr pellets prepared by pressing pellets containing 100mg KBr and 1 mg sample. Thermogravimetric analysis (TGA) of about 10 mgsamples was carried out using a Shimadzu TA-50 thermal analyzer at aheating rate of 10° C./min from room temperature to 600° C. under drynitrogen. The phase change temperature and latent heat of the sampleswere measured using a differential scanning calorimeter (DSC, Q2000).DSC measurements were conducted by heating 10 mg samples sealed in analuminum pan at a heating rate of 5° C./min under a constant stream ofargon at a flow rate of 20 mL/min.

FIG. 1B(e) shows an XRD pattern of pure PEG-6000. FIG. 1B(f) illustratesthe sample shown pure in FIG. 1A(c), but impregnated with PEG-6000. FIG.1B(g) presents sample illustrates the sample shown pure in FIG. 1A(d),but impregnated with PEG-6000. The composites of PEG-6000 and Ca²⁺-dopedMgCO₃ in FIGS. 1B(f) and 1B(g) indicate diffraction peaks in the 20range of 15°30°, ascribed to characteristic diffractions of crystallinePEG-6000. The XRD patterns of the PEG-6000/10 mol % Ca²⁺-doped MgCO₃ orPEG-6000/15 mol % Ca²⁺-doped MgCO₃ composites included the diffractionpeaks of both PEG-6000 and the MgCO₃ material, but the peak intensitiesof MgCO₃ were weaker compared with those of pure PEG-6000. Thus, theresults illustrated in FIG. 1A(a) to 1B(g) confirm that the compositePCM was composed of PEG and MgCO₃, and no new peaks were observed.

FIG. 2A presents FT-IR spectra of (a) MgCO₃, (b) 5 mol % Ca²⁺-dopedMgCO₃, (c) 10 mol % Ca²⁺-doped MgCO₃, and (d) 15 mol % Ca²⁺-doped MgCO₃,each synthesized as described herein. The peaks at 3442 cm⁻¹ areattributed to the stretching vibration of water molecules and a hydroxylgroup. This peak was not observed in MgCO₃ alone, and the 3442 cm⁻¹ peakwas not observed even after the addition of 15 mol % Ca²⁺ to MgCO₃. Thespectrum of neat MgCO₃ exhibited strong absorption bands at 1476, 885,and 748 cm⁻¹, assigned to the asymmetric C—O stretching vibrations ofthe anhydrous MgCO₃ phase. The weak absorption band at 855 cm⁻¹ wasattributed to the bending modes of CO₃ ²⁻.

Interactions between PEG and the supporting inorganic materials werecharacterized by FT-IR spectroscopy at room temperature, as shown inFIG. 2A(b). The FT-IR spectrum of PEG-6000 showed an absorption band at3468 cm⁻¹ attributed to OH stretching vibrations. PEG-6000's spectrumincluded peaks at 2889 cm⁻¹ attributed to the aliphatic C—H stretching,peaks at 1464 and 1339 cm⁻¹ attributed to C—H bending vibrations, andpeaks at 1278 and 1095 cm⁻¹ attributed respectively to O—H and C—O—Hstretching vibrations. Pure PEG-6000 includes a peak at 1109 cm⁻¹attributable to a C—O—C stretching vibration.

FIG. 2B(f) to (i), showing the composite PEG-inorganic spectra, revealseveral peaks similar to those observed in the pure PEG spectralpattern. Some absorption peaks visible in the spectra of thePEG/(Ca)MgCO₃ composite PCM may be due to strong interactions and/orpenetration of PEG into the (Ca)MgCO₃ support. Some shifts in absorptionpeaks can be observed, indicative of chemical bonding between thebridging oxygen atoms of Mg²⁺ and CO₃ ²⁻ and the hydroxyl group of PEG.This interaction may prevent leakage of liquid PEG from the CaMgCO₃matrix.

Melting temperatures of inorganic supporting materials can be reducedthrough encapsulating an organic PCMs (e.g., palmitic acid) intoinorganic shells, e.g., AlOOH, potentially due to the strong interfaceinteractions between the core and shell. New peaks observed at 509 cm⁻in the spectra can be attributed to vibrations/stretching ofcoordination bonds between the CaMgCO₃ nanoparticles and the hydroxylgroups of PEG. The FT-IR data show that that PEG strongly interacts withthe porous matrix. PEG and CaMgCO₃ displayed outstanding chemicalcompatibility, as confirmed by FT-IR.

FIG. 3A shows FE-SEM images of MgCO₃ prepared as described herein,PEG-impregnated MgCO₃, FIG. 3B 5 mol % Ca²⁺-doped MgCO₃, FIG. 3C 10 mol% Ca²⁺-doped MgCO₃, and FIG. 3D 15 mol % Ca²⁺-doped MgCO₃. FIG. 3Aindicates that the MgCO₃ product was composed of rhomboidal particleswith an average length of 15 μm and thickness of 5 μm. The surfacemorphology of the single-crystal MgCO₃ was smooth and did not changeafter calcination at 400° C. for up to 2 hours. Average rhomboidal MgCO₃crystal length may be in a range of from 1 to 20, 2.5 to 19, 5 to 18,7.5 to 17.5, 10 to 17, 12.5 to 16.5, or 14 to 16 μm and/or an averagerhomboidal MgCO₃ crystal thickness may be in a range of from 0.5 to 10,0.75 to 9, 1 to 8, 2.5 to 7.5, or 4 to 6 μm. Ratios of rhomboidaldimensional length: height of MgCO₃ crystals within the scope of theinvention may be in a range of from 1:10 to 10:1, 1:8 to 8:1, 1:6 to6:1, 1:4 to 4:1, 1:2 to 2:1,and even 1:1. Together with the L:H ratio,or separately, ratios of rhomboidal dimensional length and/or height:thickness of MgCO₃ crystals within the scope of the invention may be ina range of from 1:2 to 20:1, 1:1 to 10:1, 1:1 to 8:1, 1:1 to 7:1, 2:1 to6:1, or 2:1 to 5:1, with the upper and lower ratio end points beingapplicable to any combination. The rhomboidal shapes may be triangular,square or rectangular, hexagonal, and/or trapezoidal prismatic ordiamond prismatic, i.e., having acute angles in a range of from 85 to15, 75 to 25, 60 to 30, or 50 to 40° and corresponding obtuse angles.Moreover, the crystal shape(s) may independently include 0, 1, 2, 3, 4,5, 6, 7, or 8 right angle corners. Crystals may have pyramidal shapes 3,4, or 5-faces or more. Up to 25, 20, 15, 10, 7.5, 5, or 2.5% of theMgCO₃ crystals may take on an at least partially monoclinic, triclinic,cubic, orthorombic, and/or hexagonal crystal structure, depending ondoping. Increased doping may lead to dopant-type-dependant amorphism.The MgCO₃ crystals may be at least 50, 60, 70, 75, 85, 90, or 95%rhomboidal, with remainders being largely amorphous and/or spheroidal.The porosity of the inorganic material, particularly the MgCO₃ crystalsor the Ca²⁺-doped MgCO₃ crystals, may be in a range of 1 to 33, 2 to 25,5 to 20%.

The method described herein predominantly yielded uniform rhombohedralparticles, i.e., in an amount of at least 75%, or at least 80, 85, 90,92.5, 95, 97.5, 98, 99, 99.5, or even 99.9%. However, large or smallmicroparticles can also be produced by the hydrothermal method describedherein in an amount of, e.g., no more than 30, 20, 12.5, 10, 8, 6, 5, 4,3, 2, or 1% based on the total amount of particles. The particle sizeranged from the hundreds of nanometers scale to tens of micrometers, andthe mean size in the longest dimension of these microparticles may be ina range of from 50 nm to 50 μm, 100 nm to 40 μm, 250 nm to 35 μm, 500 nmto 30 μm, 750 nm to 25 μm, 1 to 20 μm, 2.5 to 17.5 μm, 5 to 15 μm, 7.5to 12.5 μm, or even 10 μm. The crystallinity of the microparticleslarger than 100 nm improved significantly during the hydrothermalprocess. This produced clean, smooth, single-crystalline MgCO₃. However,rhomboidal building block particles disintegrated upon doping with Ca²⁻during the hydrothermal process. The Ca²⁺ ions are believed to functionas a hammer, forming very fine powders with a porous structure. FE-SEMimages in FIG. 3(b) to (d) reveal that, after doping with at least 5 mol% Ca²⁺, the MgCO₃ particles began to break apart. Doping with 15 mol %Ca²⁺ converted all building blocks into a flake-like morphology.

FIG. 4 shows the pore size distributions of 10 mol % Ca²⁺-doped MgCO₃(with circular points) and 15 mol % Ca²⁻-doped MgCO₃ (square points)samples characterized by N₂ adsorption and desorption isotherms. Amarked leap at the high P/P₀ range (0.8 to 1.0) was observed in theisotherms of both the 10 mol % Ca²⁺-doped MgCO₃ and 15 mol % Ca²⁺-dopedMgCO₃ samples, indicating a pore size distribution that ranged fromlarge mesopores to macropores. A small average pore size tends to hinderPCM molecular motion, which decreases the latent heat storage capacityof the material. Conversely, large pores provide insufficient capillaryforces to retain the liquid wax. A mesoporous support may thus performbest in certain applications. Therefore, the PEG-based composite PCMsstabilized by mesoporous matrices may provide high-performance heatstorage systems.

FIG. 5 shows that a broaden pore size distribution was obtained over therange 15 to 150 nm, showing the presence of numerous large mesoporesand/or macropores in the 15 mol % Ca²⁺-doped MgCO₃ sample. Exemplarywidths at half-heights of pore size distributions for 10 mol %Ca²⁺-doped MgCO₃ may be in a range of from 0.01 to 60, 0.1 to 57.5, or 1to 55 nm, while that of 15 mol % Ca²⁺-doped MgCO₃ may be from 0.01 to110, 0.1 to 100, 1 to 95, 5 to 90, or 10 to 85 nm. The 15 mol %Ca²⁺-doped MgCO₃ sample displayed a large specific surface area of 15.47m²/g compared to that of the bulk MgCO₃ 8.14 m²/g. While the pore sizeis not limited beyond the particular application, inventive (5 to 20,esp. 15 mol %) Ca²⁺-doped MgCO₃ may have specific surface area in arange of from 5 to 50 m²/g, or at least 7.5, 10, 11, 12, 12.5, 13, 14,or 15 m²/g and/or up to 45, 40, 35, 30, 27.5, 25, 22.25, or 20 m²/g.FIG. 5 shows numerous pores several tens of nanometers in diameterpresent on the 15 mol % Ca²⁺-doped MgCO₃ sample surface, possibly as aresult of MgCO₃ nanoparticle etching. The 15 mol % Ca²⁺-doped MgCO₃sample shows a Barrett-Joyner-Halenda (BJH) adsorption average porediameter of 20 nm and a BJH adsorption cumulative pore volume of 0.16cm³/g. Porous 15 mol Ca²⁺-doped MgCO₃ material thus may provide a goodsupport matrix for shape-stabilized composite PCMs, and may have anaverage pore diameter in a range of from 1 to 40, 5 to 35, 10 to 30, 15to 25, 17.5 to 22.5 nm and/or a BJH adsorption cumulative pore volume ina range of from 0.05 to 0.30, 0.075 to 0.275, 0.10 to 0.25, 0.125 to0.225, 0.13 to 0.20, 0.14 to 0.18, or 0.15 to 0.175 cm³/g.

FIG. 6A shows characteristic XPS peaks for the magnesium, calcium,oxygen and carbon elements on the surface of the 15 mol % Ca²⁺-dopedMgCO₃. FIG. 6B shows the Mg 1s core level spectrum was resolved intothree components peaks. The high binding-energy component peak in FIG.6B may be assigned to magnesium hydroxide, while the second intensitypeak may be attributed to Mg, and the third to magnesium oxide (MgO).

The calcium 2p core level XPS spectrum, shown in FIG. 6C, consists oftwo sublevels (2p_(3/2) and 2p_(1/2)) due to spin-orbit splitting. Thebinding energies in the calcium 2p region, may be determined bydevolution of the spectrum using a non-linear least squares algorithmwith a Shirley background and a Gaussian-Lorentzian mixed line shape.The resolution of the calcium 2p spectrum into its component peaks isshown in FIG. 6C.

FIG. 6D shows a high-resolution scan of oxygen 1s XPS spectrum can bedivided into three component peaks, attributed to the CaO and thehydroxyl group bonded with magnesium (MgO), as indicated on the plot.The oxygen 1s high binding-energy component may be assigned to adsorbedoxygen species (such as water and carbonates) to Mg and Ca that resultfrom exposure of the sample to air as well as due to hydrothermalprocess. FIG. 6E shows a high-resolution scan of carbon 1s XPS spectrum.

FIG. 7 shows TGA curves of pure PEG (0 to 100 mass %), 15 mol %Ca²⁺-doped MgCO₃ (upper, less weight loss), and the composite ofPEG-6000/15 mol % Ca²⁺-doped MgCO₃ (middle). The TGA decompositionprocess was carried out at a heating rate of 5° C./min under an argonatmosphere. TGA analysis showed that the 15 mol % Ca²⁺-doped MgCO₃powders decomposed into CO₂ and MgO at 530° C. A weight loss of 34.5%was determined for the porous 15 mol % Ca²⁺-doped MgCO₃ with heating upto 500° C. due to the removal of the absorbed water and hydroxyl groups.Pure PEG-6000 began to melt at about 440° C., and the total weight losspercentage was 100% at 500° C. The weight loss of the composite PCM mayhave resulted from the removal of PEG molecules from the composite, inaddition to the removal of the absorbed water and hydroxyl groups fromthe matrix. The removal temperature and/or melting temperature of PEGfrom the composite was 20% higher than the corresponding values of purePEG, showing the presence of good interactions between PEG and the 15mol % Ca²⁺-doped MgCO₃ matrix. These results show that the porous 15 mol% Ca²⁺-doped MgCO₃ support improves the thermal stability of PEG bycreating a defensive barrier. The total weight loss percentage ofPEG-6000/15 mol % Ca²⁻-doped MgCO₃ composites was 78.2% upon heating upto 500° C., but may be, for example no more than 50, 60, 70, 75, 77.5,80, 85, or 90%. The impregnation ratio of PEG (M) in the composite couldbe calculated from the residual weight percent of the composite (W) andthe pure 15CaMgCO₃ (n) using Equation 5.(1−M)×n=W  (Eq. 5)In the example shown in FIG. 7, the PEG impregnation ratio of thecomposite can be calculated as 73.0%, though the impregnation ratio maybe tailored as desired, and may be in a range of from 60 to 85, 65 to80, or 70 to 75%.

FIG. 8A illustrates the melting-freezing DSC curves obtained from purePEG and FIG. 8B shows that of PEG-6000/10 mol % Ca²⁺-doped MgCO₃composite. The PEG DSC curve indicates a melting point (T_(m)) of 63.84°C. and a freezing point (T_(f)) of 39.50° C., as seen in FIG. 8A. Thephase change characteristics of the composite were similar to those ofpure PEG. The phase change enthalpies of the samples tested werecalculated based on the enclosed area under the DSC curve during themelting process. The melting enthalpy of the pure PEG was 221.3 J/g, andthe PEG-6000/10 mol % Ca²⁺-doped MgCO₃ composite had a melting enthalpy(η_(apparent,) including the mass of 10 mol % Ca²⁺-doped MgCO₃) of 159.3J/g, a value lower than that of pure PEG, probably due to a weightfraction of the porous 10 mol % Ca²⁺-doped MgCO₃ matrix in thecomposite, as seen in FIG. 8B. These DSC results make sense based on theheterogeneous morphology of the powdered 10 mol % Ca²⁺-doped MgCO₃, andthe imperfect PEG penetration and/or mixing. As a result, the meltingand solidification peaks in the DSCs had shoulders around the narrowpeaks.

The microstructure of the PEG-6000/15 mol % Ca²⁺-doped MgCO₃ compositeis shown in FIG. 9A by FE-SEM The porous texture of the 15 mol %Ca²⁺-doped MgCO₃ composite matrix is not clearly visible in FIG. 9A, andthe top surface appears compact and flat because the pores of the spongyand/or flake-like particles are filled with PEG. This indicates that theporous structure of the 15 mol % Ca²⁺-doped MgCO₃ matrix is occupiedand/or filled with a large quantity of PEG, most likely forming anefficient shape-stabilized PCM. As seen in FIG. 9B, the PEG-6000/15 mol% Ca²⁺-doped MgCO₃ composite sample displayed an apparent meltingenthalpy (η_(apparent), including the mass of 15 mol % Ca²⁺-doped MgCO₃)of 426.2 J/g, higher than the value obtained from pure PEG, probably dueto the lack of weight fraction in the porous CaMgCO₃ matrix in thecomposite.

FIG. 10A to 10C show DSC heating curves of 10A 15 mol % Ca²⁺-dopedMgCO₃, 10B PEG-6000 alone, and 10C composite PEG-6000/15 mol %Ca²⁺-doped MgCO₃. Unlike conventional inorganic PCMs, with high thermalconductivity and energy storage densities over 530 J/g (Li₂CO₃ andNa₂CO₃ composite PCMs), or 357 J/g (LiNO₃), or 222 J/g (NaNO₃), theapparent chemical bonding between bridging oxygen atoms of Mg²⁺ and CO₃and hydroxyl group(s) of PEG in organic-inorganic composites within thescope of the invention may exhibit synergistic activity between organicand inorganic materials and consequently a high latent heat. As seen inFIG. 10A pure PEG-6000 exhibits two endothermic peaks, at around 65° C.and 430° C. respectively. As seen in FIG. 10B the 15 mol % Ca²⁺-dopedMgCO₃ sample alone exhibits an intense endothermic peak 580° C. As seenin FIG. 10C, the composite PEG-6000/15 mol % Ca²⁺-doped MgCO₃ hasendothermic peaks at 62° C., 450° C., and 550° C., which means that the15 mol % Ca²⁻-doped MgCO₃ is significantly involved in the phase changeenthalpy. Higher phase change enthalpies of certain inventiveencapsulated composites may correlate to higher filling rates of PEG inthe microcapsules. Known AlOOH composites exhibit no such endothermicpeak in the same temperature range of the DSC scan. Therefore,surprisingly, CaMgCO₃ composites can contribute to phase changeenthalpy, not found in at least certain forms of AlOOH composites. Thelow phase change enthalpy of known microcapsules using AlOOH may be dueto low filling rates of palmitic acid in the microcapsules.

Composite PCMs according to the invention can have unexpectedly highmelting enthalpies. Such high latent heats have not previously beenreported in the literature, and composite PCMs generally had lowerlatent heats than pure PCMs unless the supporting material provides asynergetic latent heat effect. Equation 6 can quantify the totalweight-related energy storage capacity of the composites herein,including PEG-6000/15 mol % Ca²⁺-doped MgCO₃, which gives an theapparent thermal storage efficiency (η_(apparent)) of 183% for the 15mol % Ca²⁺-doped MgCO₃ sample. Apparent thermal storage efficiencies ofcomposites within the scope of the invention may be in the range of from125 to 250, 140 to 225, 150 to 210, 160 to 200, or 175 to 190%.η_(apparent)=(ΔH _(apparent) ΔH _(apparent))×100%  (Eq. 6)

FIGS. 11A and B show photographs of the results of seepage tests of thePEG-6000/15 mol % Ca²⁺-doped MgCO₃ composite carried out by maintainingthe material at a temperature slightly above the melting temperature ofPEG alone and observing the stability of the structure. FIG. 11A showspure PEG and row FIG. 11B shows the PEG-6000/15 mol % Ca²⁺-doped MgCO₃composite, each heated at 80° C. over the indicated time periods. Asshown in FIG. 11A, pure PEG completely melts into a liquid after heattreatment at 80° C. for 7 min or longer. However, the PEG-6000/15 mol %Ca²⁺-doped MgCO₃ composite retains its solid powder form during thecomplete heating process, and no liquid leakage is observed from thesample. The porous 15 mol % Ca²⁺-doped MgCO₃ structure is believed toprovide mechanical strength to prevent seepage of molten PEG duringphase transformation.

FIG. 12A shows the microstructure and FIG. 12B shows the phase changeproperties of the PEG-6000/15 mol % Ca²⁺-doped MgCO₃ composite PCM afterthe seepage test, measured using FE-SEM and DSC. FIG. 12A evidences thatthe microstructures of inventive composites, and their phase changeproperties, can remain unchanged after the seepage test. Thus, the PEGin the 15 mol % Ca²⁺-doped MgCO₃ matrix retained its original structureand properties after undergoing thermal cycling, as seen in the DSCcurves in FIG. 12B. These thermal cycling tests revealed that thePEG/CaMgCO₃ composites provided excellent thermal reliability over atleast 50 melting/solidifying cycles. The properties of inventivecomposites may be maintained over at least 75, 100, 150, 200, 250, oreven 500 cycles, though the cycling will be limited by the harshness ofthe heating conditions as well as oxidative and other degradation of thecomposite, particularly the organic portion. The results in FIG. 12Bdemonstrate that PEG/CaMgCO₃ has enhanced thermal reliability, with noimpurity peaks being observed in the DSC curves of the sample, evenafter 50 cycles.

FIG. 13A (a) to (c) illustrate the compatibility of the 15 mol %Ca²⁺-doped MgCO₃ composite PCMs with the preserving materials. That is,the 15 mol % Ca²⁺-doped MgCO₃ composite PCMs were coated onto (a) Al,(b) Sn, and (c) Cu metal sheets. The Al and Sn sheet samples in contactwith the inventive PCMs displayed almost no corrosion, indicating that15 mol % Ca²⁺-doped MgCO₃ can have good compatibility with Al or Snsurfaces, particularly under high-intensity solar radiation and/or highhumidity. On the other hand, the Cu sheet metal samples displayedcorrosion in regions contacted by 15 mol % Ca²⁺-doped MgCO₃, as seen in(C). FIG. 13B (a′) to (c′) show the same materials after two months(July-August) under environmental conditions in Dammam, Saudi Arabia.The initially white Cu sample, after coating with the 15 mol % Ca²⁺doped MgCO₃PCM, changed to a light blue color under the same thermalcycling conditions as the Al or Sn samples. This advises additionalcoatings and/or barriers for copper or Cu alloy storage media using the15 mol % Ca²⁺-doped MgCO₃ composite PCMs.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

The invention claimed is:
 1. A method of preparing a phase changecomposite of polyethylene glycol (PEG) encapsulated in a porous matrix,the method comprising: dissolving magnesium nitrate, calcium nitrate,and ammonium carbonate in water to form a solution; stirring thesolution at constant pH in the range of 7.5 to 9.5 for 6 to 18 hours;heating the solution at a temperature in the range of 150 to 250° C. for12 to 48 hours to produce mesoporous calcium ion doped magnesiumcarbonate comprising 5-15 mol % calcium ions on MgCO₃; mixing themesoporous calcium ion doped magnesium carbonate with an alcoholicsolution comprising a polyethylene glycol having a number-averagemolecular weight of from 4,000 to 8,000 and an alcohol, and stirring fora time in the range of 2 to 8 hours; and evaporating the alcohol toproduce the composite of the polyethylene glycol encapsulated in amesoporous calcium ion doped magnesium carbonate.
 2. The method of claim1, wherein the mesoporous calcium ion doped magnesium carbonatecomprises at least 90 wt % calcium ion doped magnesium carbonate.
 3. Themethod of claim 1, wherein the composite has an apparent meltingenthalpy in a range of from 400 to 600 J/g.
 4. The method of claim 1,wherein the mesoporous calcium ion doped magnesium carbonate has a BETspecific surface area in a range of from 10 to 20 m²/g.